introduction lab chemistry

SIMPLE DISTILLATION OF TOLUENE/CYCLOHEXANE MIXTURE

OBJECTIVES: to distill toluene and cyclohexane and to determine the composition of the distillate using Gas Chromatography (GC) analysis.

TIME ALLOCATED: ONE LAB PERIOD

Introduction

Distillation: Is the separation of a liquid mixtures in its components exploiting different boiling points. The boiling point is defined as the temperature at which the vapor pressure of a liquid is equal to the external pressure applied to the surface of the liquid. In an open container this external pressure is atmospheric pressure. A phase change (from liquid to

gas) occurs when a compound in the distilling flask, upon heating, reaches its boiling point. When the gas vapors encounter the condenser of the distilling apparatus, the compound reverts back from gas to liquid and it is collected in the collecting flask, known as receiver

There are four main laboratory scale distillation techniques:

1) Simple distillation: the vapor is immediately collected into a condenser therefore the distillate is not pure but its composition is identical to the composition of the vapors at the given temperature and pressure. (see below for details about distillation of a mixture and its composition). Simple distillation is utilized when the boiling points differ from each other at least 40°C or when you need to separate liquids from non-volatile solids or oils.

2) Fractional distillation: it is used to separate liquids whose boiling points differ of about 2-3°C. It involves repeated vaporization-condensation cycles using a packed fractionating column. When the lower part of the distilling column is maintained at a higher temperature than the upper part of the column, the condensate will be partially re-vaporized as it flows down the column. As the original condensate is re-

vaporized, the composition of the vapor changes and each cycle will lead to a solution enriched In the component with lower boiling point. Whereas, the condensate that flows down the column correspondingly becomes richer in the less volatile component. This repetitive process is equivalent to performing a number of simple distillations within the column. Each of these vaporization/condensation cycles is known as a theoretical plate. During a fractional distillation of a binary liquid mixture, the head temperature (the temperature at the top of the column) ideally should rise to the normal boiling point of the more volatile component and remain there until that component is completely removed. The head temperature may then drop somewhat, indicating that the more volatile component has been removed. If additional heat is provided, the less volatile component will begin to distill, and the now higher head temperature should remain constant until all of the second component has distilled.

3) Steam Distillation: Is a technique utilized for compound that are heat sensitive and it involves bubbling steam In a mixture of compounds. It Is widely used for extracting essential oils from plants.

4) Vacuum distillation: It Is utilized to lower the boiling points of the compounds to distillate. In fact, reduced pressure reduces the boiling points of compounds. It can be used for liquids with very high boiling points, but also for compounds that could decompose easily. The technique is used in an apparatus that you use daily in lab: the rotary evaporator.

Characterization by GC Gas chromatography, in particular gas liquid chromatography, is a technique utilized to separate and characterize small volatile organic molecules. This method Is based on the partition of the components, originally in the gas phase, between a moving inert gas phase and a stationary phase (column). Separation is achieved in the column by different strength of interaction of the analytes with the stationary phase. Each compound will have a specific retention time (Rt), that is defined as the time required by the analyte to pass through the column and It Is calculated from the time of the Injection to the time of detection. What you obtain Is a chromatogram. The area for each peak in the chromatogram is proportional to the amount of compound in the mixture.

Example

Figure 1. Distillation apparatus scheme

Figure 4. Reaction of nitrogen mustard with deoxyguanosine via intramolecular SN2

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Example calculation set up for % cyclohexane in this sample:

(561.5241/771.6500) x 100

For % toluene, you can either subtract the percent cyclohexane from 100 or calculate as shown below:

(210.1259/771.6500) x 100

Behavior of a mixture of two miscible (soluble) liquids. According to Dalton’s Law, the total vapor pressure exerted by a mixture of two liquids is given by the sum of the partial pressures exerted by the individual components. Using Rauolt’s Law, the pressure exerted by the individual components is equal to the product of the vapor pressure of the component times its mole fraction. Combining Rauolt’s and Dalton’s Law yields:

PT = XAPA + XBPB

where PT=total pressure

XA=mole fraction of compound A

PA=vapor pressure of compound A

XB=mole fraction of compound B

PB=vapor pressure of compound B

When a mixture boils, the composition of the vapor is different from the composition of the liquid. This is the key to distillation. At a given temperature the vapor phase is richer in the more volatile component than is the boiling liquid with which the vapor is in equilibrium.

MATERIAL AND METHODS

• 1:1 mixture of cyclohexane: toluene

• Distillation apparatus

• Boiling chips

• Round bottom flask

EXPERIMENTAL

During this experiment you will distillate cyclohexane and toluene from a 1:1 mixture using simple distillation.

1. Obtain 5 mL of 1:1 mixture of cyclohexane/toluene and transfer it to a pre- weighed 10 mL round bottom flask.

2. Add a boiling chip to the flask and assemble the distillation apparatus (check with the instructor before starting the distillation).

3. Place a pre- weighed receiver vial at the end of the condenser side arm before heating to minimize the vapors released to the lab. Lower the distillation apparatus into the heating mantle and heat the sample. (Make sure the heating mantle is plugged into the Variac setting!)

4. Continue heating the sample until distillation begins. Remember that:

i. The distillation rate should be slow with a steady distillation rate of 3-4 drops per minute.

ii. The rate of distillation may be controlled by slowly increasing the variac settings.

iii. You need to maintain a constant distillation rate.

5. Use 3 vials to collect 3 1.5 mL fractions.

6. As the distillation starts, collect about 1.5 mL of distillate in each test tube. Record the temperature at which you collect the fractions. you can stop the distillation when you have about 0.5 ml In the round bottom flask. To stop the distillation, raise the flask out of the sand.

7. Record GC traces for the three fractions and determine the composition.

NOTE You never perform distillation on a close system as it could explode

Real life application

Distillation, in particular steam distillation is used to separate essential oils from plants. For example, methyl butyrate is associated with pineapple smell, cinnamaldehyde with cinnamon, and methone with minty flavor.

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Key words: simple distillation, fractional distillation, vacuum distillation, steam distillation, distillate, vapors, condenser, theoretical plates, mobile phase, stationary phase.

POST-LAB ASSIGNMENT

• Complete the work sheet assigned

• Calculate the composition of the three fractions collected. (attach your GC chromatogram)

• Complete the writing assignment "Introduction" following the template.